The microquasar Cygnus X-3 underwent a giant radio flare in April 2017, reaching a maximum flux of ∼ 16.5 Jy at 8.5 GHz. We present results from a long monitoring campaign carried out with Medicina at 8.5, 18.6 and 24.1 GHz, in parallel to the Metsahovi radio telescope at 37 GHz, from 4 to 11 April 2017. We observe a spectral steepening from α = 0.2 to 0.5 (with Sν ∝ ν −α) within 6 h around the epoch of the peak maximum of the flare. We further study the radio orbital modulation of Cyg X-3 emission associated with the 2017 giant flare and with six mini-flares observed in 1983, 1985, 1994, 1995, 2002 and 2016. The enhanced emission episodes observed during the decline of the giant flare at 8.5 GHz coincide with the orbital phase φ ∼ 0.5 (orbital inferior conjunction). On the other hand the light curves of the mini-flares observed at 15 − 22 GHz peak at φ ∼ 0, except for the 2016 light curve (obtained with VLBI observations including SRT) which is shifted of 0.5 w.r.t. the other ones. We attribute the apparent phase shift to the variable location of the emitting region along the bent jet. This might be explained by the different accretion states of the flaring episodes (the 2016 mini-flare occurred in the hypersoft X-ray state).

Published in The Astrophysical Journal, Volume 906, Issue 1, id.10, 9 pp.